Ubuntu 12.04 has Swift in its "main" archives (version 1.4.8-0ubuntu2.2) which was superseding the PPA version anyway. Could you tell us what your current swift version is ("dpkg -l swift") ? If the main version was used, you can just remove the PPA (sudo add-apt-repository -r ppa:swift-core/release).

If the PPA is not installed, you can just skip the add-apt-repository line in the instructions.

The add-apt-repository was meant to add a PPA which would contain the swift packages. Since Swift was included in the "main" repository for 12.04, there is no need to add a PPA to install Swift. So you can just ignore the error.